Ionic liquids are used in ionic liquid ion sources. Ionic liquid ion sources are impregnated with solvent-free ionic liquids, also known as room temperature molten salts. Ionic liquids are composed of non-solvent mixtures of complex organic and inorganic cations and anions. The surface tension of ionic liquids are similar to typical organic liquids. These ions are large enough to produce a poorly coordinated mixture that remains in the liquid phase at moderate temperatures. Some of them display super-cooling tendencies in which they remain as liquids well below their nominal freezing points. Just as their inorganic cousins (i.e., simple salts such as NaCl, KBr, etc.) at their melting points (typically >850° C.), they exhibit appreciable electrical conductivity at room temperature, making them suitable for electrostatic deformation and subsequent Taylor cone formation. Ionic liquids are thermally stable over a wide range of temperatures (i.e., they do not boil, but decompose at temperatures ˜250-500° C.) and are apparently non-toxic being able to be used with applications with green standards, such as in the synthesis and catalysis of chemical reactions. Ionic liquids also can be used in electrochemical systems, such as in high energy density super-capacitors. The attractiveness in this area relies on the size of their electrochemical window (i.e., the maximum potential difference sustainable by the liquid before electrochemical reactions are triggered), which is higher than in conventional aqueous solutions. In addition to these characteristics, ionic liquids have the important property of having extraordinarily low vapor pressures at, or moderately above, their melting points. This allows them to be used in high vacuum equipment in open architectures such as liquid-impregnated needles.
There are a large number of ionic species and compositions. Ionic liquids have favorable electrochemical properties. For example, it is possible with ionic liquid ion sources to extract nearly monochromatic positive or negative ion beams from a nanometer-sized liquid source. In contrast to other organic solutions, it is possible with ionic liquids to achieve a regime where only ions are electro-sprayed. These characteristics are ideal for precision focused ion beam applications (e.g., microscopy and analysis, lithography, implantation, etching, microelectronics mask repair, plasma contactor, space propulsion, etc). Of relevance is the introduction of a compact, relatively simple source of negative ions. Negative ions have the potential to eliminate problems associated with charge build-up on dielectric or electrically floating substrates. These problems range from limiting the focusing capabilities of the focused ion beam system to surface damage due to differential charging. In addition, negative ions from some ionic liquids are chemically reactive, potentially enhancing the etching rates without recurring to chemical assistance.
Ionic liquid ion sources can be unique in producing high-brightness emission of complex organic and inorganic molecular ions. Ionic liquid ion sources are able to produce positive or negative ion beams with ideal characteristics for focused ion beam applications: (1) narrow energy distributions, (2) high brightness, (3) small source size, and (4) wide selection of liquids with very diverse molecular compositions. Ionic liquid ion sources can be used as a simple and compact source of nearly-monoenergetic negative ions, which could reduce considerably the charge build-up that limits the ability to focus non-neutralized positive ion beams onto dielectrics (insulators or some biological samples) or conductive, but electrically floating targets, and act as a chemically reactive etch agent for materials micro- and nanoprocessing applications. The dependencies of beam spot size down to the nanometer level can be determined as a function of column operating parameters and geometry. Nano-scale beam spots allow for higher resolution in focused ion beam applications (e.g., in microscopy and ion lithography). Resolution, together with the source brightness, is a desirable property of an ion source applicable to nanotechnology.
Implementation of ionic liquid ion sources for focused ion beam applications can have a broader impact on, for example, the preparation of samples for transmission electron microscopy (TEM) when used as a milling tool, in high-resolution ion microscopy of a variety of conductive (electrically floating, biased or grounded) and non-conductive specimens, in ion lithography and implantation, among others. Ionic liquid ion sources can also be used in the analysis of biological samples due to favorable optical properties of negative beams when interacting with non-metallic substrates. Ionic liquid ion sources can be used with chemically reactive negative ion beams for etching applications (for instance, BF4—). Current practice with positive ions utilize the injection of a reactive species (e.g., Cl2 gas, for example) to enhance the rate of material removal. Avoiding that practice would be beneficial in the operation of UHV systems.
An existing ionic source includes a Liquid Metal Ion Source, which uses a liquid metal. Except for metals such as In or Ga, most sources work only at very high temperatures, thus introducing some difficulties in the source implementation, as the thermal evaporation rate of neutrals increases and chemical reactions with the needle material (e.g., tungsten) may occur. These thermal and compatibility issues mean that only a relatively small number of metallic elements and alloys can be successfully used in liquid metal ion sources. In addition, operation with liquid metals can be possible only in the positive polarity, thus emitting positively charged metallic ion beams.
Another existing ionic source includes capillary-based electrosprays, which includes a capillary tube. An example of such an electrospray is described in U.S. Pat. No. 7,199,364 entitled “Electrospray ion source apparatus” by Thakur. Capillary based electrosprays can be used to produce highly charged micro and nano-droplet beams. The flow of ionic liquids used in capillary emitters are controlled by line hydraulics, back pressure and/or applied voltage to the emitter. Bubbles and small particles can be problematic with these this type of electrospray. Droplets, however, may not give enough fuel efficiency as a thruster to be competitive with other forms of propulsion. The specific charge of emitted species can be increased by increasing the liquid conductivity and decreasing the supply flow rate, however, some ionic solutions (e.g., Formamide saturated with NaI) exhibit copious ion emission but most of the mass is still emitted as droplets, decreasing the thruster efficiency. In contrast, bubbles and liquid starvation (e.g., vapor lock) is less likely with ionic liquid ion sources.